X-ray imaging is a consolidated technology that has reached high levels of excellence during more than a century of refinement. Conventional x-ray imaging systems include many individual components that function together to produce high quality radiographic images with the lowest possible radiation dose. Many x-ray imaging systems use a solid, stationary, beam-shaping filter—traditionally called a static bow-tie filter (see, e.g., FIG. 1A)—to differentially attenuate the central region and the periphery of the x-ray beam to compensate for the shape of the human body (or other object) cross-section, which is usually thicker in the center and thinner in the sides as seen from the x-ray source/focal spot. A traditional static bow-tie filter can serve several function, for example: reduction of the dose to the patient by avoiding unnecessary overexposure of the side of the body, reduction of the scatter coming from the overexposed areas, optimization of the detector dynamic range by avoiding saturation of the detector response in the periphery of the body, and generation of a uniform quantum noise level at substantially every pixel of the image (and correspondingly a uniform signal-to-noise-level).
The ideal shape of a static bow-tie filter is determined by the shape and material composition of the imaged object and the x-ray energy spectrum to be used. Imaging systems using a static bow-tie filter are therefore optimized to operate exclusively with a particular object size, shape, orientation, composition, and x-ray energy, and the performance of the system is reduced if the imaging parameters deviate from the ideal design parameters. To partially address this limitation, some clinical systems provide a small set of filters for the user to choose from, such as one filter for an adult person torso, one for head scans, or one for pediatric patients. However, these application-specific filters still fail to match the attenuation profile of most patients and do not provide optimal performance.
An imaging modality in which filters are of particular interest is computed tomography. In computed tomography, a filter can significantly reduce the radiation dose and mitigate some undesirable artifacts in the reconstructed images such as cupping. However, the practical performance of a static bow-tie filter is limited because the shape of the filter has to be designed assuming that the patient has a circular cross section and a fixed diameter. In this case, the same filter is used for most patients and for substantially every angle of rotation of the x-ray source around the patient. Since none of the human body parts have a circular cross section, the performance of the system is suboptimal. A technique known as automatic exposure control (or tube current modulation in tomographic imaging) can be used to scale the x-ray intensity used at each individual projection, and therefore compensate for the different maximum attenuation at different angles. However, this technique cannot correct for the different object profiles at different angles. The performance of a static bow-tie filter is further degraded in clinical practice for patients that are not perfectly centered on the axis of rotation of the scanner, which corresponds to the center of the symmetry of a static bow-tie filter.